Bone anchoring device for stabilising bone segments and seat part of a bone anchoring device

ABSTRACT

A bone anchoring device for stabilizing bone segments is provided, with a bone anchoring element which has a shank part and a head, a receiver part for articulated connection of the bone anchoring element to a rod, wherein the receiver part has a first end and a second end opposite this, a bore extending from the first end in the direction of the second end for guiding through the shank part, adjacent to the bore a section for receiving the head, an opening extending from the second end in the direction of the first end for inserting the shank part with head and a recess extending from the second end in the direction of the first end for forming a channel for receiving the rod, forming two open legs, wherein the receiver part has two slit-shaped recesses extending at the front from the floor of the channel in the direction of the first end.

FIELD OF THE INVENTION

The invention relates to a bone anchoring device for stabilising bonesegments with a bone anchoring element and a seat part (receiver part)for articulated connection of the bone anchoring element to a rod and areceiver part of a bone anchoring device of this kind according to thepreambles of patent claims 1 and 2.

BACKGROUND OF THE INVENTION

From DE 37 11 013 C1 a pedicle screw for stabilising spinal columnsegments is known, in which the screw element is articulatedly connectedto a receiver part. The receiver part comprises two halves surroundingthe head of the screw element and a guard ring holding them together.The receiver part further has receiving slits for inserting a threadedrod. Locking the head in the receiver part and fixing the threaded rodis done by nut-shaped fixing screws screwed on to the threaded rod onboth sides of the receiver part, which fixing screws act laterally onthe receiver part.

A bone anchoring device according to the preamble of patent claim 2 anda receiver part according to the preamble of patent claim 1 are knownform DE 39 23 996 C2. In the known device, to fix the angle of the bonescrew relative to the receiver part, a pressure element acting on thehead of the bone screw is provided. Fixing the rod, constructed as athreaded rod, takes place by means of nuts screwed on to the rod, whichact laterally on the receiver part. In the assembled state a force isexerted on the head in the axial direction of the receiver part via thepressure element in such a way that the head is locked in the receiverpart.

A bone anchoring device according to the preamble of patent claim 2 anda receiver part according to the preamble of patent claim 1 in whichthere is a pressure element for fixing the head are also known from DE43 07 576 C1. Fixing the head and rod takes place by means of a fixingscrew which can be screwed into the receiver part and a lock nut screwedon to the receiver part, which exert a force on the pressure element inthe axial direction of the receiver part.

In the U.S. Pat. No. 5,728,098, a bone anchoring device according to thepreamble of claim 2 and a receiver part according to the preamble ofclaim 1 are described. Head and rod are fixed separately by two rings ofmemory shape alloy. For this purpose, the receiver part has a pair ofopposite upper slots and a pair of opposite lower slots. Receiver part,screw and lower ring are pre-assembled loosely, and in this state, thebone anchoring device is screwed into the bone. Thereafter the rod isinserted and the upper ring is mounted. By the temperature of the bodythe two memory shape alloy rings contract. Thus, head and rod can befixed separately in the receiver part by the two corresponding rings.

SUMMARY OF INVENTION

The object of the invention is to provide an improved bone anchoringdevice or a receiver part for same, wherein fewer components arerequired, which are easier for the user to handle and can be configuredas more filigree from the point of view of dimensions.

The object is achieved by a receiver part according to patent claim 1and a bone anchoring device according to patent claim 2. Furtherdevelopments of the invention are cited in the subordinate claims.

In the bone anchoring device according to the invention extremely secureclamping is produced by a combination of pressure forces actinglaterally and in the axial direction of the receiver part.

Further features and advantages of the invention emerge from thedescription of embodiment examples using the figures.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a bone anchoring device according to a first embodiment inthe assembled state illustrated in partial section.

FIG. 2 shows the bone anchoring device of FIG. 1 in perspective explodedillustration.

FIG. 3 shows the bone anchoring device of FIG. 1 illustrated in partialsection in the unloaded state.

FIG. 4 shows the bone anchoring device of FIG. 1 in the loaded state.

FIG. 5 shows the detail I of FIG. 4 in enlarged schematic illustration.

FIG. 6 shows a modified embodiment of the bone anchoring device of FIG.1 illustrated in partial section.

FIG. 7 shows an enlarged perspective sectionalised illustration of anelement of the bone anchoring device according to FIG. 6.

FIG. 8 shows a second embodiment of the bone anchoring device.

FIG. 9 shows an enlarged illustration of a detail II of a modificationof the embodiment of FIG. 7.

FIG. 10 shows a side view of a modified bone anchoring device.

FIG. 11 shows a perspective view of the modified bone anchoring deviceof FIG. 10.

FIG. 12 shows a partially sectional view of the bone anchoring device ofFIG. 10 in a state in which the inner screw is not yet fully tightened.

FIG. 13 is a schematic representation of the fixed state of the boneanchoring device according to FIG. 12 with the inner screw fullytightened.

FIG. 14 is a perspective view of the receiver part of FIGS. 10 and 11.

DETAILED DESCRIPTION OF INVENTION

As can be seen from FIGS. 1 to 4, in a first embodiment the boneanchoring device has a screw element 1 with a thread section 2 with abone thread and a head 3. The head 3 is constructed as shaped like asegment of a sphere adjacent to the thread section. Coaxially to thethread shaft and on the end opposite the thread section the head 3 has arecess 4 for bringing into engagement with a screwing tool.

The bone anchoring device further contains a receiver part 5,substantially constructed as cylindrical. The receiver part has a firstend 6 and a second end 7 opposite it. Provided adjacent to the first end6 is a bore 8 aligned as axially symmetrical, the diameter of which islarger than that of the thread section 2 and smaller than that of thehead 3. The receiver part 5 further has a coaxial second bore 9, whichis open on the end 7 of the receiver part opposite the first bore 8 andthe diameter of which is large enough for the screw element 1 to beinserted through the open end with its thread section and its head.Between the first bore 8 and the second bore 9 is provided a coaxialsection 10, which is immediately adjacent to the first bore 8 and isconstructed in the shape of a segment of a sphere with a radius which issubstantially identical to the radius of the section of the head 3shaped like a segment of a sphere when the bone anchoring device isassembled and fixed. Because it is constructed in the shape of a segmentof a sphere, the bore 9 is narrowed in such a way that section 10 formsa spherical holder for the head 3.

The receiver part 5 further has a recess 11, extending from the secondend 7 in the direction of the first end, which is substantially U-shapedand forms a channel, into which a rod 12 can be inserted. Two open legs13, 14 of the receiver part are formed by the recess 11. The recess 11is further constructed in such a way that it has a first section 11 awith a width which is dimensioned precisely so that the rod 12 can beinserted without jamming and a second area 11 b, which forms the actualchannel in which the rod 12 rests. Area 11 b contains the floor of thechannel and is constructed substantially as spherical with a widthwaysdiameter which is larger than the diameter of the rod 12. Thus, as shownin FIG. 3, there is a small intermediate space 11 c between the channelwall and the inserted rod, so the inserted rod 12 has a certain mobilitycrosswise to its longitudinal axis.

The receiver part 5 further has two slit-shaped recesses 15 opposite oneanother and extending at the front from the floor of the channel in thedirection towards the first end 6. The recesses extend to apredetermined depth into section 10 shaped like a segment of a sphere.The width of the slit-shaped recesses 15 is dimensioned in such a waythat the open legs 13, 14 are elastically movable in respect of oneanother to a preset amount.

A section with an outer thread 16 is further constructed on the receiverpart 5 in the area of the open legs 13, 14, adjacent to their open end7. The outer thread 16 is constructed as a metric thread.

A nut 17 is provided, with an inner thread which cooperates with theouter thread 16. The length of the outer thread section and themeasurements of the nut 17 in the axial direction are constructed insuch a way that in the screwed in state the nut 17 presses on the rod12.

In operation the screw element 1 is inserted from the open end of thesecond bore 9 into the first bore 8 until the head 3 with its lower sidecomes up against the upper edge of the section 10 shaped like a segmentof a sphere and forming the spherical holder. On further insertion thelegs 13, 14 are pressed apart slightly owing to their elasticity causedby the slit-shaped recess 15, so the screw head snaps into the sphericalholder 10. There it is held as freely movable and the screw can bescrewed into the bone by intervention of the screwing-in tool into therecess 4.

The rod 12 is then inserted and the nut 16 (sic) loosely screwed down.Accurate alignment of the receiver part 5 relative to the rod 12 thentakes place. In this aligned position the nut 17 is tightened firmly inthis way until it presses on the rod 12 with its end facing the rod andthus exerts a force, characterised by arrow F in FIG. 3, on the rod 12.The rod 12 is thus firmly clamped in the channel. The force F acting onthe rod via the nut 17 generates counter-force components F_(G)originating from the floor of the channel and illustrated by arrowsF_(G) in FIGS. 3 and 4. As illustrated in FIG. 5, this counter-forceacts via the rod on the lower edge of the nut 17, whereby via thethreaded flanks of nut and receiver part a force component Fi directedinwards (as illustrated in FIG. 4) is generated. Further screwing downof the nut 17 results in these counter-force components F_(G) being ofsuch a size that the legs 13, 14 undergo a force directed inwards ofsuch a size that they are pressed together, this being possible owing totheir elasticity caused by the slit-shaped recesses 15. The pressingtogether of the legs 13, 14 causes the spherical holder 10 to closefirmly round the head shaped like a segment of a sphere and lock it inits position. This is illustrated in FIG. 4 by the radial forcecomponents F_(B) acting on the spherical head and locking it. As can beseen in particular in FIGS. 3 and 4, the channel, spread out in thenon-loaded state of the device, narrows in such a way that the rod 12rests against the channel wall 11 b.

Overall reliable fixing of rod and head is achieved with one singlefixing element in the form of the nut 17.

In FIGS. 6 and 7 a modified embodiment of the bone anchoring deviceaccording to FIGS. 1 to 4 is shown which differs from the former in theholder of the head 3 in the receiver part 5.

Identical parts are therefore described by the same reference numerals.

Receiver part 50 differs from receiver part 5 in that adjacent to thefirst end it has a first bore 80, the diameter of which is slightlylarger than the diameter of the head 3 shaped like a segment of asphere. The bore 80 widens over a small conical area 81 into a bore 90extending as far as the second end 7.

Instead of the section 10 shaped like a segment of a sphere in thepreviously described embodiment, forming the holder for the head 3, inthis embodiment the holder for the head 3 is formed by a separateinsertion part 60.

The insertion part 60 is substantially constructed as hollow-cylindricalwith a maximum outer diameter which is slightly smaller than the innerdiameter of the bore 90, so part 60 can be inserted into the receiverpart 90 starting from its second end 7. The insertion part 60 has afirst end with a section 61, chamfered or tapering conically on theouter side, the angle of taper of which corresponds to the angle oftaper of section 81 of the receiver part 50, so in the inserted stateconical section 61 of insertion part 60 rests on conical section 81 ofthe receiver part.

On its inner side the insertion part 60 is constructed adjacent toconical area 61 with a section 62 shaped like a segment of a spheresimilar to section 10 of receiver part 5 of the previously describedembodiment, the radius of which substantially corresponds to the radiusof the head 3. Section 62 shaped like a segment of a sphere forms theholder for the head 3.

The insertion part 60 further has on its end opposite conical section 61two slit-shaped recesses 63, offset from one another by 180°, whichextend to a predetermined length into section 62 shaped like a segmentof a sphere. The width and depth of the recesses 63 are dimensioned insuch a way that, when the insertion part 60 is inserted into thereceiver part 50, the recesses 63 come to overlap with the slit-shapedrecesses 15. The length of the insertion part 60 in the axial directionis less than the distance of the floor of the channel from the first end6 of the receiver part 50.

Between the insertion part 60 and the inner wall of the receiver partthere is further provided a safety device against twisting, notillustrated in the figures, so that the slit-shaped recesses 15 of thereceiver part and the insertion part are not displaced in respect of oneanother while the screw element is being screwed into the bone. A safetydevice against twisting of this kind can be implemented, for example, bycountersunk bores provided on the circumferential side of the insertionpart 60, which cooperate with corresponding crimped bores on thereceiver part.

In operation the screw element is first inserted into the insertion part60 with its thread section 2 in the direction of conical section 61until the head 3 snaps into the holder 62, made possible owing to theelasticity produced by the slit-shaped recesses. The insertion part 60is then inserted into the receiver part 50 together with the screwelement until conical section 61 of the insertion part rests on conicalsection 81 of the receiver part. The screw element is then screwed intothe bone. Alignment of the receiver part and fixing the head 3 and rod12 takes place via the nut 17, as in the first embodiment. The legs 13,14 of the receiver part, moved towards one another, in this embodimentexert a clamping force on the head via the slit insertion part 60.

In a modification the slit-shaped recesses 63 of the insertion part aredeeper than the recesses 15 of the receiver part 50. This makesinsertion of the screw element easier owing to the greater elasticity ofthe insertion part, while the receiver part can be configured assufficiently stable.

The embodiment containing the insertion part 60 has on the one hand theadvantage that the receiver part 50 is easier to manufacture than in thefirst embodiment and on the other hand previous insertion of the screwelement makes handling easier during application.

The second embodiment illustrated in FIGS. 8 and 9 and its modificationdiffers from the embodiment illustrated in FIGS. 1 to 7 in separatefixing of head 3 and rod 12.

In the embodiment illustrated in FIG. 8 the receiver part 5 isconstructed like the receiver part according to FIGS. 1 to 5.

Instead of the nut 17 a cap 71 is provided, which has an inner thread 72cooperating with the outer thread 16 of the receiver part 5. The cap 71further has a central bore 73 and a cylindrical projection 74, extendingtherefrom in the direction of the open edge, with an inner thread 75 forreceiving a rod fixing screw 76. The cylindrical projection 74 has adiameter which is slightly smaller than the inner diameter of bore 9 ofthe receiver part, so when the cap is screwed on the cylindricalprojection slides into the receiver part and there is sufficient playtowards the inner wall of the receiver part, so pressing together thelegs to clamp the head is possible as in the first embodiment. Thecylindrical projection 74 further forms a stop for the legs 13, 14 insuch a way that the legs can be pressed together only far enough for therod still to be freely displaceable in the channel. The rod fixing screw76 is constructed as a set screw and has a recess 77 for bringing intoengagement with a screwing-in tool. In the fully screwed down state ofthe cap (not illustrated in FIG. 8) the cap comes up against the openend 7 of the legs 13, 14, forming a stop. The measurements of the cap 71and the length of the inner thread area of the cap are dimensioned insuch a way that in the fully screwed down state the cap does not presson the inserted rod 12.

In operation the screw element is placed into the receiver part andscrewed into the bone as in the first embodiment. The rod is theninserted and the cap 71 with screwed in rod fixing screw 76 is looselyscrewed down. Adjustment of the position of the receiver part andadjustment of the rod are still possible in this state. Fully screwingdown the cap as far as the stop, like screwing down the nut according tothe first embodiment, causes the legs of the receiver part to be pressedtogether, leading to clamping of the head 3 in the holder, while the rodis still longitudinally displaceable in the channel. The rod is thenfixed by further screwing in of the rod fixing screw 76 until itsunderside presses on the rod.

In FIG. 9 a modification of this embodiment is shown. The receiver part5 is constructed as described previously, with the difference that onthe end of the outer thread 16 facing the first end 6 of the receiverpart in the outer wall of the receiver part a circulating diagonal face78 is provided, which is aligned in the direction towards the first end6 at an angle of approximately 30° to the central axis M of the receiverpart.

Adjacent to the open edge of the cap 71, correspondingly provided on theinner side is a diagonal face 79 which cooperates with the diagonal face78 of the receiver part in the screwed down state of the cap. The widthof the diagonal face 79 of the cap is slightly smaller than the width ofthe diagonal face 78 of the receiver part, so when the cap is screweddown the faces can slide on one another to a certain extent.

In operation, in this modified embodiment when the cap is screwed downuntil it comes up against the open end of the legs the diagonal faces 78and 79 end up on top of one another, wherein an additional force isexerted on the legs in the direction towards the central axis M of thereceiver part, which presses the legs together.

In the embodiment illustrated in FIGS. 8 and 9 it is also possible touse the insertion part 60 according to the embodiment illustrated inFIGS. 6 and 7.

In the figures the slit-shaped recesses 15 of the receiver part 5 or 50and the insertion part 60 are shown with a rectangular floor. However,they can also be constructed as rounded.

The invention has been described using an example of a bone screw.Instead of the bone screw hooks can be provided, wherein this is thenconstructed in two parts and has a head and a hook section which can beconnected thereto, which is not connected until the head has beeninserted.

The term rod is used above as representative of all rod-shaped elementsand also includes these.

FIGS. 10 to 14 show a modified bone anchoring device. Parts which areidentical to the embodiments shown in FIGS. 1 to 9 are characterizedwith the same reference numerals. The bone anchoring device includes thescrew element 1 with a thread section 2 with a bone thread and aspherical segment-shaped head 3. As in the previous embodiments the head3 has a recess 4 for bringing into engagement with a screw driver.

The receiver part 150 has a first end 6 and a second end 7 opposite tothe first end. In an assembled state the first end 6 is directed towardsthe screw element 1. As shown in FIG. 12, the receiver part has—as inthe previous embodiments—a coaxial bore 8, the diameter of which islarger than that of the thread section 2 and smaller than that of thehead 3. Further, the receiver part 150 has a coaxial second bore 9 whichis open at the second end 7. The diameter of the second bore 9 is largeenough to allow the screw element 1 to be inserted through the open end.Adjacent to the first bore 8 a coaxial spherically-shaped section 10 isprovided which has a radius substantially identical to the radius of thesection of the head 3 when the bone anchoring device is assembled.

The receiver part 150 further has a recess 11, extending from the secondend 7 in the direction of the first end 6. The recess 11 issubstantially U-shaped and forms a channel with a bottom 111 into whichthe rod 12 can be inserted. The width of the channel is just as largethat the rod can be inserted without jamming and fits into the channelwhen it is not fixed. By the recess 11 two legs 13, 14, which are opentowards the second end 7, are formed. The legs 13, 14 comprise an innerthread 160 to allow an inner screw 170 to be screwed between the legs13, 14.

The receiver part 150 has two slit-shaped recesses 151 which are open atthe first end 6 and extend in the direction towards the second end 7 andwhich are located on opposite sides in the direction of the longitudinalaxis of the channel. The slit-shaped recesses 151 end at a distance fromthe bottom 111 of the channel. The length and the width of theslit-shaped recesses 151 are dimensioned such that a lower part 51 ofthe receiver part 150 has a certain elasticity to be able to clamp thehead 3 of the screw element 1 when a force is exerted which pressestogether the two portions of the lower part 51 of the receiver part 5which are separated by the slit-shaped recesses.

In operation the screw element 1 is inserted from the second end 7 untilthe head 3 rests against the section 10 of the receiver part 150. Inthis state the screw element is pivotably held in the receiver part andthe screw element can be screwed into the bone. Thereafter the angularposition of the screw element 1 with respect to the receiver part 150 isadjusted and the rod 12 is inserted into the channel. In the first stateas shown in FIG. 12 when the inner screw 170 is not yet fully tightened,there is still a possibility to adjust the position of the receiver part150 with respect to the rod 12 or with respect to the head 3. Whenfurther screwing in the inner screw 170 towards the rod 12 and finallytightening the inner screw 170, the inner screw 170 presses onto the rod12 and fixes the rod 12 in its position. During tightening of the innerscrew 170 the inner screw cooperates with the inner thread 160 of thelegs 13, 14 in such a way that a force FS is generated which tends tomove apart the legs 13, 14 from each other. Simultaneously, because ofthe elasticity of the lower part 51 of the receiver part 150 due to thepresence of the slit-shaped recesses 151, the lower part 51 of thereceiver part 151 is compressed so that a pressing force FP is generatedwhich clamps the head 3 to lock the head in its position. FIG. 13schematically illustrates the moving apart of the legs. The forcesrequired to clamp the head are not as big that the inner screw isloosened due to splaying of the legs. In practice, the inner screwremains fixed between the legs.

With the construction as described above it is possible tosimultaneously fix the rod and to lock the head in its position withoutusing a separate compression member or an outer nut. Therefore, the boneanchoring device is simple and compact.

Further modifications are possible. Instead of only two slits there maybe several slits in a circumferential direction of the lower part of thereceiver part 150. A sufficient number of slits should be provided toensure secure clamping. The number of slits is selected according to therequired clamping force. The shape of the section 10 against which thehead 3 rests, can be other than spherical.

Instead of inserting the screw element 1 through the opening on top ofthe receiver part 150, it is also possible to design the receiver partin such a way that the screw element can be inserted from the loweropening 8. In this case the dimensions of the opening and the number ofthe slits have to be adapted such that it is possible to snap thereceiver part 150 onto the head 3 of the screw element 1.

1. A receiver part for articulated connection of a bone anchoring element with a rod having a diameter, the bone anchoring element comprising a shank part and a head, the receiver part comprising: a first end and a second end opposite the first end; a bore extending from the first end in the direction of the second end for guiding through the shank part; adjacent to the bore, a section for receiving the head; an opening extending from the second end in the direction of the first end for inserting the shank part with head; a recess extending from the second end in the direction of the first end for forming a channel with a bottom for receiving the rod, the recess forming two open legs; and two slit-shaped recesses extending from the bottom of the channel in the direction of the first end, wherein the width of the channel with the rod inserted is larger in an unloaded state than the diameter of the rod.
 2. The receiver part in accord with claim 1, further comprising a device that cooperates with the two open legs to press the legs together.
 3. The receiver part in accord with claim 1, wherein the section for receiving the head has a shape corresponding to the head to be received.
 4. The receiver part in accord with claim 1, wherein the head has a shape of a segment of a sphere and the section for receiving the head has a corresponding shape to cooperate with the head.
 5. The receiver part in accord with claim 1, further comprising a separate insertion part provided in the receiver part for holding the head, wherein the insertion part comprises corresponding slit-shaped recesses that overlap with said two slit-shaped recesses in the receiver part.
 6. The receiver part in accord with claim 2, wherein the two open legs have an outer thread and the device that cooperates with the two open legs comprises a nut with an inner thread that cooperates with the outer thread on the two open legs.
 7. The receiver part in accord with claim 6, wherein the outer thread and the nut each comprise the dimensions that are chosen in such a way that the nut can be screwed down to such an extent that it presses on an inserted rod.
 8. The receiver part in accord with claim 6, wherein the nut is structured and arranged in such a way that, in the fully screwed down state, it does not fix an inserted rod and the receiver part further comprises a rod fixing screw for fixing the rod.
 9. The receiver part in accord with claim 8, wherein the outer thread of the two open legs and the inner thread of the nut each comprise a cooperating portion whereby a pressure force an be exerted on the two open legs in a direction perpendicular to the slit-shaped recess.
 10. The receiver part in accord with claim 8, wherein the nut is structured and arranged in the form of a cap having a central bore and a cylindrical projection surrounding the central bore and having an inner thread for receiving the rod fixing screw.
 11. A bone anchoring device for stabilizing bone segments, the device comprising: a bone anchoring element comprising a shank part and a head; a receiver part for articulated connection of the bone anchoring element to a rod; the receiver part comprising: a first end and a second end in the direction of the second end for guiding through the shank part; adjacent to the bore, a section for receiving the head, an opening extending from the second end in the direction of the first end for inserting the shank part with head; a recess extending from the second end in the direction of the first end for forming a channel with a bottom for receiving the rod, the recess forming two open legs; and two slit-shaped recesses extending from the bottom of the channel in the direction of the first end, wherein the width of the channel with the rod inserted is larger in an unloaded state than the diameter of the rod.
 12. The bone anchoring device in accord with claim 11, further comprising a device that cooperates with the two open legs to press the legs together.
 13. The bone anchoring device in accord with claim 11, wherein the section for receiving the head has a shape corresponding to the head to be received.
 14. The bone anchoring device in accord with claim 11, wherein the head has a shape of a segment of a sphere and the section for receiving the head has a corresponding shape to cooperate with the head.
 15. The bone anchoring device in accord with claim 11, further comprising a separate insertion part provided in the receiver part for holding the head, wherein the insertion part comprises corresponding slit-shaped recesses that overlap with said two slit-shaped recesses in the receiver part.
 16. The bone anchoring device in accord with claim 12, wherein the two open legs haven an outer thread and the device that cooperates with the two open legs comprises a nut with an inner thread that cooperates with the outer thread on the two open legs.
 17. The bone anchoring device in accord with claim 16, wherein the outer thread and the nut each comprise the dimensions that are chosen in such a way that the nut can be screwed down to such an extent that it presses on an inserted rod.
 18. The bone anchoring device in accord with claim 16, wherein the nut is structured and arranged in such a way that, in the fully screwed down state, it does not fix an inserted rod and the receiver part further comprises a rod fixing screw for fixing the rod.
 19. The bone anchoring device in accord with claim 18, wherein the outer thread of the two open legs and the inner thread of the nut each comprise a cooperating portion whereby a pressure force can be exerted on the two open legs in a direction perpendicular to the slit-shaped recess.
 20. The receiver part in accord with claim 18, wherein the nut is structured and arranged in the form of a cap having a central bore and a cylindrical projection surrounding the central bore and having an inner thread for receiving the rod fixing screw.
 21. A receiver part for articulated connection of a bone anchoring element with a rod having a diameter, the bone anchoring element comprising a shank part and a head, a receiver part comprising: a first end and a second end opposite the first end; a bore extending from the first end in the direction of the second end for guiding through the shank part; adjacent to the bore, a section for receiving the head, an opening extending from the second end in the direction of the first end for inserting the shank part with head; a recess extending from the second end in the direction of the first end for forming a channel with a bottom for receiving the rod, the recess forming two open legs; and two slit-shaped recesses extending from the first end in the direction of the second end, said slit-shaped recesses being provided on opposite sides of the receiver part in the direction of the longitudinal axis of said channel; wherein the head is locked in its position by the action of a force which tends to move apart said legs.
 22. The receiver part of claim 21, further comprising a device that cooperates with the two open legs to exert a force on said legs which tends to move apart said legs.
 23. The receiver part of claim 22, wherein said legs comprise an inner thread and said device is an inner screw to be screwed between said legs.
 24. The receiver part of claim 21, wherein said section to receive said head has a shape corresponding to the shape of head to be received.
 25. The receiver part of claim 21, wherein the length of said slit-shaped recesses in the direction of the first end to the second end is dimensioned such that the head can be clamped in the section for receiving the head.
 26. The receiver part of claim 21 further comprising one or more additional slit-shaped recesses extending from the second end in the direction of the first end.
 27. The receiver part of claim 21, wherein the width of the channel corresponds to the diameter of the rod to be inserted.
 28. A bone anchoring device for stabilizing bone segments, the device comprising: a bone anchoring element comprising a shank part and a head; a receiver part for articulated connection of the bone anchoring element to a rod; the receiver part comprising: a first end and a second end opposite the first end; a bore extending from the first end in the direction of the second end for guiding through the shank part; adjacent to the bore, a section for receiving the head, an opening extending from the second end in the direction of the first end for inserting the shank part with head; a recess extending from the second end in the direction of the first end for forming a channel with a bottom for receiving the rod, the recess forming two open legs; and two slit-shaped recesses extending from the first end in the direction of the second end and being located on opposite sides in the direction of the longitudinal axis of said channel; wherein the head is locked in its position by the action of a force which tends to move apart said legs.
 29. The receiver part of claim 28, further comprising a device that cooperates with the two open legs to exert a force on said legs which tends to move apart said legs.
 30. The receiver part of claim 29, wherein said legs comprise an inner thread and said device is an inner screw to be screwed between said legs.
 31. The receiver part of claim 28, wherein said section to receive said head has a shape corresponding to the shape of the head to be received.
 32. The receiver part of claim 28, wherein the length of said slit-shaped recesses in the direction of the first end to the second end is dimensioned such that the head can be clamped in the section for receiving the head.
 33. The receiver part of claim 28 further comprising one or more additional slit-shaped recesses extending from the second end in the direction of the first end.
 34. The receiver part of claim 28, wherein the width of the channel corresponds to or is slightly smaller than the diameter of the rod to be inserted. 